This invention pertains to a laser type source of coherent luminous radiation, more particularly to the reduction in the number of longitudinal and/or transverse modes (spectral and/or spatial fine-tuning) of such a source.
In order to reduce the emission beam width of a laser, there have been proposed in the art selective elements requiring adjustment such as Lyot and Fabry-Perot filters.
Various spectral fine-tuning devices have been published or patented.
Known in the prior art, for example, is EP 284908 describing a device for controlling or regulating an emission wavelength xcexe and an optical power emitted by a semiconductor laser. The optical power emitted by the laser is transmitted at least in part to an optoelectrical detector device and at least to an optical filtration device that is selective in terms of wavelength. Part of the power transmitted to this filtration device is transmitted to another optoelectrical detector device. According to this state of the art, the semiconductor laser, the filter device and the detectors are integrated on a common substrate. The integrated filter device is constituted by a Bragg grating or by a directional coupler or an interferential filter or by a unit formed by two or more Bragg gratings arranged one behind each other in the direction of transmitted power propagation and/or by directional optical couplers and/or by interferential filters.
There have been proposed in the state of the art wavelength-selective devices located in the laser cavity but which are not self-adapting. The article xe2x80x9c82 nm of continuous tunability for an external cavity semiconductor laserxe2x80x9d published in Electron. Lett. 27, 183 (1991) describes a fine-tuning and stabilization process for an extended cavity laser diode using a conventional diffraction grating. The diffraction grating is a selective element of the extended cavity. The two factors which contribute to the fine-tuning are the dispersion of the grating and the length of the extended cavity which reduces the Shalow-Townes width of the emitted beam.
Also known are laser diodes of the DFB (distributed feedback) type or DBR (distributed Bragg reflector) type integrating a Bragg grating in the active zone (DFB) or as a cavity mirror (DBR). These Bragg mirrors are not self-adapting.
Various solutions are known in the state of the art for the spectral stabilization of a laser by means of an external cavity. U.S. Pat. No. 4,907,237 describes a process for the stabilization of a laser diode by means of an external cavity that has a particular resonance frequency. A part of the output beam of the external cavity is reinjected into the laser cavity.
Self-adapting external fine-tuning systems have also been proposed in the prior art. The article xe2x80x9cWavelength-stable, narrow-spectral-width oscillation of an AlGaInP diode laser coupled to a BaTiO3:Co stimulated photorefractive backscattering phase conjugatorxe2x80x9d published in Appl. Phys. B 65, 329(1997) describes a laser employing a phase conjugation mirror using the Bragg selectivity of the gratings by reflection and reinjecting a part of the beam into the laser cavity. The device has a crystal placed outside of the cavity. The phase relation is thus fixed.
Also known is EP 433122 which describes a ring cavity laser device. It involves light injection from a master laser to a slave laser which thereby inherits the coherence properties of the master laser. The presence of the photosensitive material does not modify the spectral properties of the master laser because there is no feedback from the slave cavity to the master cavity.
The article xe2x80x9cLaser frequency bandwidth narrowing by photorefractive two-beam couplingxe2x80x9d published in Opt. Lett. 17, 481 (1992) describes filtering the bandwidth frequency at the exit of the laser by a photorefractive grating inscribed by the beams exiting from the laser cavity.
The document OPTICS LETTERS, Vol. 12, No. 2, Feb. 1, 1987, pages 117-119, authored by Whitten et al. xe2x80x9cMode selection in a continuous-wave dye laser with an intracavity photoreactive elementxe2x80x9d describes a source of coherent luminous radiation comprising a linear resonant cavity and an amplifier medium placed in the resonant cavity, as well as a dynamic photosensitive material placed in the resonant cavity to form a self-adapting spectral and/or spatial filter.
The devices of the prior art exhibit various disadvantages. When the characteristics of the cavity vary, for example, due to a change in the length due to thermal variations, the efficacy of the interferential filters proposed in certain devices of the prior art is degraded.
The solutions comprising the use of an external means often require delicate adjustments and are sensitive to mechanical stresses.
This invention relates to a laser type source of coherent luminous radiation including a resonant cavity, an amplification medium placed in the resonant cavity and a dynamic photosensitive material which is placed in the resonant cavity to form a self-adapting spectral and/or spatial filter, characterized in that the cavity is a ring laser cavity and in that the dynamic photosensitive material is placed at the intersection of two beams.